The present invention relates generally to ink jet printing systems for large scale graphic generation and deals more particularly with a method for producing a large scale color graphic that presents substantially the same spectral content to a viewer when the graphic is illuminated with front or back lighting.
Large scale color graphic systems for producing images on sheet material or other substrates for use in outdoor advertising, road side billboards and signs and displays in railway or airport terminals, shopping malls and other such public areas are disclosed in U.S. Pat. No. 4,547,786 entitled "INK JET PRINTING SYSTEM" and U.S. Pat. No. 4,811,038 entitled "INK JET PRINTING SYSTEM AND DRUM THEREFORE" both of which are assigned to the same assignee as the present invention and the disclosures of which are incorporated herein by reference.
The large scale color graphic systems, such as those referenced above, produce images using four color process pigment inks on an opaque white substrate. The images are generally optimized for outdoor viewing in natural light by day and may be illuminated from the front by artificial lighting means for viewing at night. Although there is a difference in the spectral content of the viewed image under natural and artificial illumination, the lighting is sufficiently similar to cause the image produced with the four color process pigmented inks to appear substantially the same under either natural or artificial illumination. Additionally, the contrast of the image, that is, the ratio of the light reflected from the lightest area on the image to the light reflected from the darkest area on the image, remains substantially constant regardless of the illumination type.
In order to achieve a more dramatic nighttime effect, images are produced which are intended for backlight viewing, that is, the light source is located such that the image is located between the light source and a viewer. Generally, these images are produced using four color process pigmented inks deposited on a translucent white substrate. The substrate carrying the image is suspended on a frame or other such apparatus and is illuminated with a light source located at the rear, that is, on the opposite side of the substrate carrying the image.
A general problem associated with images produced on a substrate intended for backlighting is that the image must generally be produced with more highly pigmented inks or thicker ink films than those images produced on substrates intended for lighting by front illumination. When such images are illuminated by backlight at night, the image is of generally good quality however, during daytime viewing, the same image generally appears very dark and the colors may not be true, that is, not having the same spectral content when viewed by a viewer under the different lighting conditions.
It is therefore a general aim of the present invention to substantially overcome the problems associated with images produced on a substrate and which have different spectral content when illuminated by backlighting and front lighting.